Lifeguard Bosco McAuley, 20, was able to reach her in seconds, but she had taken on so much water that by the time they came ashore she was nearly unconscious.
Mr McAuley, who was patrolling in the surf because of the large number of rips and and people in the water, noticed a group around the woman raise the alarm.
"She couldn't swim, so she didn't start panicking she just went straight under and then I saw she was in difficulty and that's when I swam over to her," he told the Herald.
"At first I just saw her on a body board and it looked like she was okay, but then there were people that had their hand up. I thought they were just waving to shore; I had to take another look to realise something wasn't right."
Mr McAuley was due to end his patrol at 6pm, 30 minutes later.
"It was pure luck that I happened to be out there on water patrol, otherwise she could have died."
Trust Waikato Sunset Beach Lifeguard Service president Malcolm Beattie said the woman's condition deteriorated once she was brought ashore.
"She seemed fine, she stood up and was talking, but the next minute she collapsed.
"We had all our oxygen and portable equipment there so put her on oxygen immediately to really get her responses up and stable, but then she was starting to slip out of consciousness," Mr Beattie said.
The woman was flown to Middlemore Hospital by Westpac Rescue Helicopter.
Her family told rescuers she was unable to swim and had been warned to stay within her depth.
"She's among 500 other people out there in the water all having fun," Mr Beattie said. "It wasn't until she drifted away, took on water and got slightly panicked that some other people said, 'oh this isn't right' and our guard spotted it."
In Northland, two men blown 2km into Bream Bay in a child's inflatable dinghy yesterday were calm when help arrived, despite being without oars or lifejackets.
The men, in their 20s or early 30s and visiting from Auckland for the day, were swept out from the beach about 2.30pm.
Their families contacted Ruakaka police, who sought help from the Ruakaka Surf Lifesaving Patrol and notified Coastguard and other emergency services.
Ruakaka patrol captain Tania Ahrens said four lifeguards - Briar Taylor, Connor Cooper, and Tania's children Maysha and Joel Ahrens - went out in two inflatable rescue boats and found the men 2km offshore about 3.15pm.
They towed them back to shore, where the pair were given advice on water safety by police. Maysha Ahrens said the men were calm but very happy to see rescuers.
"They didn't really realise the trouble they were in ... they were on their new little Christmas present, a little inflatable raft, no lifejackets, no oars.
"They were just floating away, waiting for someone to come and get them, having a chat to each other."
An average of 1200 rescues are made annually in New Zealand. Last year's drowning toll was 81.
Surf Life Saving New Zealand project manager Matt Warren urged people to swim between the flags, in groups and look out for children. He said conditions could change quickly and people should remain vigilant.
Last night rescuers were scouring the Wellington Harbour for a fisherman, aged in his 40s, who fell from his boat into the water while collecting his nets on the Hutt River.
Police said the river was running high and discoloured.
- Additional reporting Nicholas Jones
The figures
Surf Lifesaving Northern Region
* Monday, December 30
Rescues: 2
First aid: 4
Preventive actions: 248
* Tuesday, December 31
Rescues: 7
First aids: 11
Preventive actions: 894
* Wednesday, January 1
Rescues:15
First aids: 11
Preventive actions: 1058
* Thursday, January 2
Rescues: 33
First aids: 51
Preventive actions: 1109
* Saturday, January 4
Rescues: 6
First aids: 6
Preventive actions: 468
Source: Surf Life Saving New Zealand.
